Centrosomes are the major microtubule nucleating organelles in most animal cells. They nucleate and organize microtubules for spindle assembly during mitosis and establish microtubule arrays in interphase cells for numerous cellular functions. Centrosomes are comprised of two major structural elements, centrioles and the pericentriolar material/centrosome matrix. Centrioles are microtubule barrels present as pairs in each centrosome, and appear to organize the pericentriolar material (Bobinnec et al., J. Cell Biol., 143:1575–1589, 1998) and anchor microtubules (Chretien et al., J Cell Biol., 120:117–133, 1997; Piel et al., J Cell Biol., 149:317–330, 2000). The pericentriolar material nucleates the growth of new microtubules and serves as a scaffold for molecules that regulate fundamental cellular processes.